Methods for operating fuel cells fed with a gas containing hydrogen and carbon oxide, and devices relating thereto

Abstract

The invention describes a method and the relevant devices for permitting an operation stable with time of a stack of membrane fuel cells fed with air and a gas containing hydrogen and at least 100 ppm of carbon monoxide, characterised in that it provides for an oxidising condition at the anodes of said cells in a self-adjusting continuous way or sequentially discontinuous way. Different embodiments of the invention are illustrated wherein the oxidising condition is obtained by adding to the gas containing hydrogen and carbon monoxide, oxygen produced in an electrolysis cell integrated to the fuel cell stack, or by flowing air through porous areas obtained in the bipolar plates or by resorting to high gas diffusion rate membranes, wherein the air flow rate is regulated in both cases by adjusting the pressure differential existing between air and the gas containing hydrogen and carbon monoxide. According to a further embodiment of the method of the invention, the oxidising condition is obtained by sequentially short-circuiting the fuel cell stacks provided with external contacts connected to a control unit.

1. A method for operating a stack consisting of a filter-press structure comprising a multiplicity of fuel cell stacks, each cell delimited by bipolar plates and comprising an anode-proton conducting membrane/cathode assembly, comprising maintaining said stack at 60-90° C., feeding the anode compartment with a fuel containing hydrogen and at least 100 ppm of carbon monoxide, adding water vapor thereto, flowing the said fuel into the cathode side of at least one membrane electrolysis cell integrated into the filter press structure of the stack, feeding the same electric current as the fuel cells, mixing said fuel with the oxygen produced in the anodic side of said at least one electrolysis cell and feeding the resulting gaseous mixture containing hydrogen, carbon monoxide and oxygen to each anode of the fuel cells, imposing an oxidizing condition to obtain under the same cell voltage a current output stable over time. 
     
     
       2. The method of  claim 1  wherein the method comprises excluding automatically said fuel feeding when the current output is interrupted. 
     
     
       3. The method of  claim 1  providing the stack with a number of fuel cells comprised between 95 and 105 for each electrolysis cell so that the molar percent of said oxygen mixed to the fuel containing hydrogen and carbon monoxide and added with water vapor is equal to 0.38-0.42 independently from the intensity of said current flowing through the electrolysis cell and the fuel cells. 
     
     
       4. The method of  claim 1  wherein said oxidizing condition is obtained by addition of air to the fuel containing hydrogen and carbon monoxide by permeation through a micro-porous area of the bipolar plates. 
     
     
       5. The method of  claim 4  wherein said micro-porous area is positioned in the upper portion of said bipolar plates. 
     
     
       6. The method of  claim 4  wherein the flow of air permeating through the porous area of the bipolar plates is controlled by regulation of a pressure differential maintained between the air fed to the cathodes and the fuel containing hydrogen and carbon monoxide fed to the anodes. 
     
     
       7. The method of  claim 4  wherein said micro-porous areas of the bipolar plates are made hydrophobic. 
     
     
       8. The method of  claim 6  wherein said pressure differential is adjusted to produce a molar percentage of oxygen in the exhaust fuel containing hydrogen and carbon monoxide equal to 0.8-1.2. 
     
     
       9. The method of  claim 1  wherein said oxidizing condition is obtained by adding air to the fuel containing hydrogen and carbon monoxide by permeation through said membranes of said assemblies. 
     
     
       10. The method of  claim 9  wherein the flow rate of said air permeating through said membranes is regulated by acting on the pressure differential maintained between the air fed to the cathodes and the fuel containing hydrogen and carbon monoxide fed to the anodes. 
     
     
       11. The method of  claim 9  wherein said membranes are high gas diffusion rate membranes. 
     
     
       12. The method of  claim 11  wherein said high gas diffusion rate membranes are proton conducting membranes containing particles of a chemically stable micro-porous material. 
     
     
       13. The method of  claim 12  wherein said micro-porous material is selected from the group consisting of silica and valve metal oxides. 
     
     
       14. The method of  claim 12  wherein said micro-porous material is a hyrophobic material. 
     
     
       15. The method of  claim 14  wherein said hydrophobic material is polytetrafluoroethylene powder. 
     
     
       16. The methodof  claim 11  wherein said high gas diffusion rate membrane has an air diffusion rate of 1 ml/hour×cm 2  at 70 to 90° C. under a pressure between 0.3 and 1 bar. 
     
     
       17. The method of  claim 1  wherein said oxidizing condition is obtained by sequentially short-circuiting fuel cell packages provided with contacts connected to an external control unit. 
     
     
       18. The method of  claim 17  wherein the number of fuel cells in said packages is lower by at least a factor of 10 with respect to the total number of fuel cells of the stack.